1. Field of the Invention
The present invention relates to semiconductor devices and more particularly to a diode which prevents the flow of electrons into a peripheral element on a semiconductor substrate and which is superior, for example, in the slope (.DELTA.I/.DELTA.V) of forward bias current.
2. Description of the Background Art
Referring to FIG. 34, a conventional diode includes an n-type buried semiconductor layer with a high donor content (which may be referred to as an "n.sup.+ buried layer " hereinafter) 503 which is formed on a p-type semiconductor substrate (hereinafter, referred to as a "p substrate ") 504. An n-type semiconductor region as a cathode region (which may be referred to as an "n-type cathode region " hereinafter) 501 is formed on n.sup.+ buried layer 503, and a p-type semiconductor region as an anode region (which may be referred to as a "p-type anode region " hereinafter) 502 is formed in the vicinity of the n-type cathode region to be in contact with p substrate 504. It is noted that diodes including the above described diode generally have a cylindrical p-type anode region which is formed on the side surface of the central n-type cathode region. Therefore, the two right and left p-type anode regions in FIG. 34 constitute the longitudinal section of one cylindrical anode.
The operation principle of the above described diode will be described in the following. The energy band of a junction between p-type anode region 502 and n-type cathode region 501 is shown in FIG. 35. In FIG. 35, a potential barrier Vo is caused at the boundary of the p-type anode region and the n-type cathode region, resulting in an energy difference eVo. The energy band chart is represented for an electron. Thus, the energy difference eVo has to be exceeded in order for electrons produced in the n-type cathode region to flow into the p-type anode region.
In the chart, Ec is energy at the bottom of a conduction band, Ev is energy at the top of a valence band, Efn is chemical potential (Fermi-energy) in the n-type cathode region, and Efp is chmical potential in the p-type anode region.
If voltage is externally applied to a diode in the energy state shown in FIG. 35, the energy band changes as shown in FIG. 36 or 37. FIG. 36 shows a case in which positive voltage, relative to n-type cathode region 501, is applied to p-type anode region 502 (forward bias), indicating that the potential barrier of a depletion layer decreases by an applied voltage V.sub.A from the level of FIG. 35 to e(Vo-Va). It facilitates hole movement from p-type anode region 502 to n-type cathode region 501 and electron movement from n-type cathode region 501 to p-type anode region 502. Thus, current flows from p-type anode region 502 to n-type cathode region 501.
On the other hand, FIG. 37 shows a case in which negative voltage, relative to n-type cathode region 501, is applied to p-type anode region 502 (reverse bias), indicating that the potential barrier of a depletion layer increases by applied voltage V.sub.A from the level of FIG. 35 to e(Vo+Va). It reduces the probability of hole movement from p-type anode region 502 to n-type cathode region 501 and electron movement from n-type cathode region 501 to p-type anode region 502. Thus, the amount of flowing current is very small. Semiconductor devices of the above described type have been improved to have the reverse bias voltage higher than an actual used voltage and widely used as clamp diodes. In other words, the semiconductor devices are used as diodes for circuit protection in case reverse bias surge voltage, for example, which exceeds an actual used voltage is suddenly applied to cathodes.
In the semiconductor device having the above described structure, application of positive voltage, relative to n-type cathode region 501, to p-type anode region 502 causes electrons to move from n-type cathode region 501 to p-type anode region 502. Electrons also move from n-type cathode region 501 to p substrate 504. Therefore, electrons flow from p substrate 504 to an element provided in the vicinity of the diode on the semiconductor substrate, contributing a malfunction of the peripheral element.
In order to solve this problem, a proposal was made to surround the entire side and bottom surfaces of an n-type cathode region by a p.sup.+ buried semiconductor layer (Japanese Utility Model Laying-Open No. 2-146458). It was found out, however, that the problems as described below occur depending on application when a region which originally functions as an anode is made a p.sup.+ region with a high acceptor content. The problems are: (a) when forward bias voltage is applied, the slope (.DELTA.I/.DELTA.V), that is, a current increase for a voltage increase is small in lower voltage, that is, the diode rectification does not occur steeply when positive voltage is applied; and (b) electrons do not move easily to the anode region and therefore the controllability of a diode by diode voltage becomes insufficient.
The following proposal was made as a semiconductor device which avoids the problems of (a) and (b) and prevents the flow of electrons into a peripheral element. In other words, two types of buried semiconductor layers, that is, p and n type buried semiconductor layers are provided between a substrate and a diode formation layer, and the resistance of a guiding region which extends upward from each of the two types of buried semiconductor layers is adjusted so as to form the reverse bias or equal potential relations between the n-type buried semiconductor layer and the p-type buried semiconductor layer (Japanese Patent Laying-Open No. 10-74958).
However, the above described structure cannot prevent the flow of electrons from the cathode through the guiding region to the semiconductor substrate, which bypasses the buried semiconductor layers. Further, adapting the above described structure makes it difficult to miniaturize semiconductor devices. Therefore, the above described structure is insufficient to prevent the flow of electrons into semiconductor substrates in the latest miniaturized and lower-voltage semiconductor devices.